1. Field of the Invention
This invention relates to the purification of crude halogenated products containing residual free bromine, by-product hydrogen bromide, catalyst, and other impurities.
2. Description of the Prior Art
Numerous processes may be employed to replace nuclear hydrogen atoms partially with bromine in aromatic compounds such as diphenyl ether. Such bromination processes in general have involved use of an up to about 20% excess of bromine in various reaction media depending upon the particular aromatic compound to be brominated. Normally such bromination is undertaken in the presence of a bromination catalyst such as iron, aluminum, and their halides, especially if a higher degree of bromination is desired.
Alternatively, partially brominated aromatic compounds may be obtained by reacting the compound in the presence of a slight excess of bromine in the absence of any reaction solvent but likewise employing a small but catalytic amount of a bromination catalyst.
Regardless of the preparation processes employed, there are produced brominated products containing undesirable amounts of occluded free bromine, by-product hydrogen bromide, catalyst residue, and other materials which adversely affect the color of the product and which may diminish its thermal stability.
High levels of purity are frequently required for brominated aromatic compounds such as partially brominated diphenyl ethers which have found utility as flame retardant agents. In particular, it is important that such brominated products have extremely low levels of residual impurities such as free bromine, bromine-containing impurities, catalyst, and the like since the presence of such impurities can have undesirable effects on the compositions in which such compounds are used as flame retardant agents or for other purposes. Purity is particularly important from the standpoint of color and thermal stability under the processing conditions to which such compounds are subjected.
Purification of halogenated products such as the brominated diphenyl ethers obtained in the foregoing manner has heretofore been attempted by a variety of work-up procedures. Thus, British Pat. No. 1,436,657 suggests that a crude product mixture of tetra- and penta-bromodiphenyl ether be worked up by treating a solution of the crude product in methylene chloride with successive washes of sodium metabisulfite, sodium carbonate, and water.
British Pat. No. 1,029,874, relating to the production of tri-brominated biphenyls (colorless liquids at room temperature), employs dilute hydrochloric acid in the product work-up. U.S. Pat. No. 2,022,634, describing the preparation of mixtures of chlorinated or brominated diphenyl ethers having more than four but less than ten halogen atoms, purifies the product by air blowing following bromine introduction and, in the case of Examples 8-10, also employs chlorobenzene solvent in combination with alternate washes with dilute hydrochloric acid and sodium hydroxide.
U.S. Pat. No. 3,192,272 purifies tribrominated dialkylbenzenes by washing first with water and thereafter with aqueous sodium carbonate followed by fractional distillation.
None of the foregoing schemes have been totally satisfactory in obtaining brominated products having required levels of purity and, accordingly, it is a primary object of this invention to obtain a process for purification of halogenated products of the character described that is superior to the techniques that have heretofore been employed.
Another object is to provide a process of the character described for producing purified partially brominated diphenyl ethers of good color and thermal stability.
A still further object is to provide a purification method of the character described that may be economically employed in obtaining purified partially brominated diphenyl ethers.